This invention relates to a logarithmic detector.
A logarithmic detector is a detector which converts an input power signal into a linear output current signal value and they are generally used in radio and radar receivers.
A conventional distributed logarithmic detector is shown in FIG. 1. The distributed logarithmic detector is a distributed device formed by a plurality of amplifiers 1, individual detectors 2 and summers 3. The amplifiers 1a, 1b are linked in series so that the input voltage across in the terminals 4 is applied as an input to amplifier 1a while the output of amplifier 1a is applied as an input to amplifier 1b and the output of amplifier of 1b is output from the distributed detector at output terminals 5. The amplifiers 1 are of the limiting output amplitude type. The detectors 2 each have a logarithmic response and the detectors 2a, 2b, 2c are arranged to detect the various voltages along the amplifier chain. Detector 2a detects the unamplified input voltage before amplification by amplifier 1a, detector 2b detects the amplified output voltage of amplifier 1a before amplification by amplifier 1b while detector 2c detects the doubly amplified output of the amplifier 1b. The outputs from the detectors 2a, 2b and 2c are all summed in summers 3a, 3b to provide a summed log output at log output terminals 6.
Distributed detectors of this type are used because the range of response of the individual detector elements 2 is generally relatively small compared to the desired range of the complete distributed detector and the use of multiple detectors with amplifier and summer chains allows the range of the distributed detector as a whole to be extended to the desired range.
Obviously the number of amplifiers, detectors and summers can be increased as required to give a desired range of response.
The overall log-linearity of the distributed detector is dependent on the log-linearity of the individual detectors and the non Log-linearity of known detectors is such that the performance of known distributed logarithmic detectors is limited by it.
Another problem with known detectors is that their log linearity is dependent on the amplification factor of the amplifier they are associated with in a distributed detector, the larger the gain the worse the log linearity. As a result to obtain good log-linearity in known distributed detectors low gains must be used, so for a large range a large number of stages are required, as well as the obvious cost disadvantage this results in a large delay as a signal passes through the distributed detector which can be a problem, particularly in radar systems.
This invention was intended to provide a logarithmic detector having an improved logarithmic linearity.